Microwave ovens generate heat in and warm up food or drink by applying microwaves to it for a predetermined period of time. That is accomplished mainly by high-frequency oscillation circuit, i.e. magnetron oscillator with its output of several hundred watts. It is known that food is heated from the outside of it by the conventional cooking appliances. On the contrary, it is heated from the inside and in a very short time when the microwave oven is used.
In such microwave ovens, it is usual that a microprocessor controls high-frequency oscillation of the ovens. In such a controlling process, the microprocessor is apt to keep the magnetron oscillator operating over a predetermined time or to make it operate irrespective of the user's intention, when it is out of order or under the influence of foreign noise(s). Such an abnormal operation of the magnetron can make food burnt and even set a fire.
It is clear that a microwave oven needs to be provided with a safeguard means which can ensure the microwave oven to operate in the exact way as the user wants and prevent an abnormal operation of it.